1. Field of the Invention
The present invention relates to a shadow mask assembly and a color cathode-ray tube (CRT) having the same and more particularly, to a shadow mask assembly having a structure to fix the shadow mask to the frame under application of a tensile force thereto, thereby keeping the mask in a specific shape, and a color CRT equipped with the assembly.
2. Description of the Related Art
Shadow-mask type color CRTs have been used extensively and most popular. In color CRTs of this type, the shadow mask is usually shaped to form part of a spherical surface. Since the shadow mask is fixed in the neighborhood of a face panel made of glass, the face panel needs to be shaped to form part of a spherical surface as well.
In recently years, there has been the tendency that the face panel is shaped to form part of a cylindrical or flat surface. This s due to the fact that the visibility of the color CRTs can be improved. In this case, it is needless to say that the face panel needs to be shaped to form part of a cylindrical or flat surface according to the shape of the mask.
However, as known well, the shadow mask is formed by an extremely thin, rectangular metal plate and has a lot of regularly-arranged small apertures for color selection and thus, its mechanical rigidity is extremely low. As a result, the mask is unable to keep its cylindrical or flat shape without application of any external force. Thus, it is usual that a proper tensile force or two is/are applied to the flat mask in a direction or two when the mask is fixed to the frame, thereby applying a proper tensile force to the mask in a desired direction to keep it in a desired cylindrical or flat shape.
An example of the structure to fix the shadow mask to the frame under application of a tensile force to the mask is disclosed in the Japanese Non-Examined Patent Publication No. 8-77936 published in March 1996, which is explained below with reference to FIG. 1.
As shown in FIG. 1, the prior-art shadow mask assembly 200 is comprised of a pair of curved bars 201a and 201b, a pair of arms 202a and 202b, and a rectangular shadow mask 203. The pair of bars 201a and 201b are fixed to the pair of arms 202a and 202b, thereby constituting the frame that supports the mask 203.
Each of the bars 201a and 201b has an L-shaped cross section and is curved to form part of a cylindrical surface. Each of the arms 202a and 202b is bent to have a shape like a U letter. The ask 203, which is curved to form part of a cylindrical surface, has a lot of regularly-arranged small apertures 204 allowing three electron beams for red (R), green (G), and blue (B) to pass through for color selection.
The two opposite ends of the mask 203 are fixed by welding to the curved top faces of the bars 201a and 201b, respectively. Thus, the mask 203 covers or closes the rectangular opening formed by the bars 201a and 201b and the arms 202a and 202b (i.e., formed by the frame). In this state, the arms 202a and 202b have elastic forces that translate respectively the bars 201a and 201b outwardly. Due to these elastic forces, the fixed ends of the mask 203 are pulled so as to be apart from each other. As a result, a tension that translate the fixed ends of the mask 203 to be apart from each other is generated in the mask 203, thereby keeping the shape of the mask 203.
According to the Publication No. 8-77936, the bars 201a and 201b and the arms 202a and 202b (which constitute the frame) are made of chromium-system stainless steel while the shadow mask 203 is made of iron.
The Publication No. 8-77936 does not disclose any fabrication method of the prior-art shadow mask assembly 200. However, it is supposed that the assembly 200 is fabricated in the following way.
First, the two ends of the arm 202a are fixed by welding to the specific positions of the bars 201a and 201b, respectively. Similarly, the two ends of the arm 202b are fixed by welding to the specific positions of the bars 201a and 201b, respectively. Thus, the bars 201a and 201b are apart from each other by a specific distance and arranged in parallel, constituting the frame having an approximately rectangular shape.
Subsequently, the bars 201a and 201b are applied with an external force or forces to be shifted nearer, thereby slightly bending the ends of the arms 202a and 202b inwardly. Next, while the ends of the arms 202a and 202b are kept bent inwardly, the two opposite ends of the mask 203 are respectively fixed to the curved top faces of the bars 201a and 201b by welding. Thereafter, the application of the external force or forces is stopped, allowing the ends of the arms 202a and 202b to return to their original positions. Thus, an outward elastic force that moves the bars 201a and 201b to be apart from each other is generated, applying a specific tensile force to the mask 203 in a direction perpendicular to the bars 201a and 201b. Due to the specific tensile force, the desired shape of the mask 203 can be kept unchanged in spite of its extremely low mechanical rigidity.
Generally, when the color CRT is operated, large part of he R, G, and B electron beams collide with the mask 203, raising he temperature of the mask 203 to near 100xc2x0 C. Due to this temperature rise, the mask 203 is partially expanded thermally to hereby induce undesired positional shift of the apertures 204. In this state, the R, G, and 5 electron beams tend to land in error on the phosphor screen located between the mask 203 and the face panel, degrading the color purity. This phenomenon is termed the xe2x80x9cdomingxe2x80x9d.
The tensile force applied to the shadow mask 203 serves not only to hold its shape but to prevent the doming phenomenon. Thus, if the mask 203 has a large thermal expansion coefficient, the tensile force needs to be increased according to the value of the coefficient.
With the prior-art shadow mask assembly 200 shown in FIG. 1, the s shadow mask 203 is made of iron and therefore, the thermal expansion coefficient of the mask 203 is as large as approximately 120xc3x9710xe2x88x927/xc2x0 C. at room temperature. Thus, to absorb the thermal expansion of the mask 203 and to mechanically hold the desired shape of the mask 203 securely, the necessary tensile force is as high as approximately 500 to 1000 kg. As a result, the bars 201a and 201b and the arms 202a and 202b need to be stronger according to the magnitude of the tensile force applied, which raises the weight and fabrication cost of the assembly 200.
Moreover, a large pressing force or forces need to be applied to the arms 202a and 202b in the welding process for the mask 203 and therefore, large-scale, expensive facilities are required for this purpose. This makes the fabrication cost of the assembly 200 higher.
Accordingly, an object of the present invention is to provide a shadow mask assembly that reduces the necessary tensile force to be applied to the shadow mask, and a color CRT equipped with the assembly.
Another object of the present invention is to provide a shadow mask assembly that reduces the weight and fabrication cost of the shadow mask assembly, and a color CRT equipped with the assembly.
Still another object of the present invention is to provide a shadow mask assembly that requires no large-scale, expensive facilities for the purpose of applying the tensile force to the shadow mask, and a color CRT equipped with the assembly.
The above objects together with others not specifically mentioned will become clear to those skilled in the art from the following description.
According to a first aspect of the present invention, a shadow mask assembly is provided, which is comprised of;
(a) a pair of bars arranged in substantially parallel at a specific distance;
the pair of bars being made of metal having a first thermal expansion coefficient;
(b) a pair of arms fixed to the pair of bars in such a way that each of the pair of arms links the pair of bars together;
the pair of arms applying a force that moves the pair of bars away from each other;
the pair of arms being made of metal having a second thermal expansion coefficient;
the pair of arms constituting a frame having an approximately rectangular opening along with the pair of bars; and
(c) a shadow mask fixed to the pair of bars so as to cover the opening of the frame;
the shadow mask being applied with a tensile force in a direction approximately perpendicular to the pair of bars due to the force of the pair of arms that moves the pair of bars away from each other, thereby keeping the mask in its specific shape;
the shadow mask being made of metal having a third thermal expansion coefficient that is low enough for suppressing the doming phenomenon;
the third thermal expansion coefficient of the mask being less than the second thermal expansion coefficient of the pair of arms.
With the shadow mask assembly according to the first aspect of the present invention, the shadow mask is made of the metal having the third thermal expansion coefficient that is low enough for suppressing the doming phenomenon and therefore, almost no tensile force is additionally applied to the mask in order to suppress the doming phenomenon. This means that the tensile force for keeping the mask in its specific shape is reduced, in other words, the necessary tensile force to be applied to the mask is lowered.
As a result, the necessary mechanical strength for the pair of bars and the pair of arms can be lowered, which reduces the weight and fabrication cost of the pairs of bars and arms (and therefore, the shadow mask assembly itself). At the same time as this, the necessary force applied to the pair of arms in the welding process can be lowered and thus, no large-scale, expensive facilities are required for the purpose of applying tensile force to the shadow mask.
In a preferred embodiment of the shadow mask assembly according to the first aspect of the invention, the first thermal expansion coefficient of the pair of bars is greater than the second thermal expansion coefficient of the pair of arms. In this embodiment, there is an additional advantage that possible disadvantages due to the thermal expansion difference between the pair of bars and the mask are difficult to occur.
In another preferred embodiment of the shadow mask assembly according to the first aspect of the invention, the first thermal expansion coefficient of the pair of bars is approximately equal to the third thermal expansion coefficient of the shadow mask. In this embodiment, there is an additional advantage that no disadvantage such as plastic deformation and/or wrinkling of the mask occur even if the shadow mask assembly is subjected to the thermal history with the highest temperature of approximately 500xc2x0 C. in the fabrication processes of color CRTs and to the operation environment with the highest temperature of approximately 100xc2x0 C. This is because almost no thermal expansion difference is induced between the pair of bars and the mask.
In still another preferred embodiment of the shadow mask assembly according to the first aspect of the invention, the metals of the pair of bars and the shadow mask are Invar (i.e., iron alloy containing nickel by 36%), and the metal of the pair of arms is at least one selected from the group consisting of iron, chromium-iron alloy, and chromium-iron-molybdenum alloy.
According to a second aspect of the present invention, a color CRT is provided, which is comprised of the shadow mask assembly according to the first aspect of the invention.
With the color CRT according to the second aspect of the invention, the shadow mask assembly according to the first aspect of the invention is included and thus, a light-weight, inexpensive color CRT with high image-quality can be realized.